1. Field of the Invention
The invention generally relates to encoders for transmitting audio information formatted in accordance with the multichannel television sound (or xe2x80x9cMTSxe2x80x9d) standard in xe2x80x9csurround soundxe2x80x9d and, more particularly, relates to encoders for xe2x80x9csurround soundxe2x80x9d transmission of modified MTS formatted audio information.
2. Description of Related Art
The MTS standard governs the transmission of audio information to televisions. In accordance with this standard, audio information is carried in two signals, the Lxe2x88x92R signal and the L+R signal. The Lxe2x88x92R signal, which represents the difference between left (or xe2x80x9cLxe2x80x9d) and right (or xe2x80x9cRxe2x80x9d) channels, carries stereo information within a first frequency band and surround sound information within a second, higher, frequency band. Due to signal attenuation and noise considerations, most devices used today to deliver audio information to a television are unable to successfully transmit the surround sound information contained in the Lxe2x88x92R band to the television. Thus, in our prior application, U.S. patent application Ser. No. 08/918,754 filed Aug. 27, 1997, we disclosed a modulator, a device which is coupled between the video and audio signal generator and the television, which enables the transmission of surround sound information to an audio reproduction device associated with the television.
While it is contemplated that the video and audio signals to be transmitted to the television may be generated by a wide variety of devices, one such device is a digital video disc (or xe2x80x9cDVDxe2x80x9d) player which reads video and audio information encoded onto the surface of a DVD and transmits the information to the television. However, because the DVD player produces video images of very high quality, the motion picture industry has long expressed concern that DVD players could be used to produce unauthorized copies of movies of quality comparable to the originals. As a result, DVD players are equipped with a copy protection feature commonly referred to as xe2x80x9cMacrovision DVD copy protectionxe2x80x9d. When a copy protected DVD disc is played back, the DVD player varies the format of the encoded video signal to create a series of modifications thereto. These signal modifications are designed to be transparent on original program viewing. However, they cause unauthorized copies made on video cassette recorders (or xe2x80x9cVCsxe2x80x9d) to be substantially degraded.
Copy protection techniques which create modifications to the video signal have, however, produced certain undesirable side effects. Specifically, the modulators used to enable the transmission of surround sound to televisions use the video signal to synchronize the audio signal being processed thereby. However, the aforementioned copy protection techniques typically modify the frequency of the horizontal synchronization signal embedded within a video signal. For example, the Macrovision DVD copy protection technique raises the frequency of the video signal from 15.734 KHz to 15.790 KHz. The resulting clocking error produces a significant amount of distortion in the audible sound reproduced using the audio signal output through the modulator.
Thus, what is needed is a device which is both capable of transmitting surround sound information to a television while preventing the copy protection feature of the signal generator from distorting the audio signal transmitted thereby. It is, therefore, the object of the invention to provide such a device.
In one embodiment, the present invention is of an audio adapter for coupling a video/audio signal generator to an audio reproduction system. The audio adapter includes a first audio input, a second audio input and a video input for coupling the audio adapter to the video/audio signal generator and an audio output which couples the audio adapter to the audio reproduction system. The audio adapter further includes an audio signal processor which generates a composite audio signal clocked to a selected frequency from a left audio signal received from the first audio input and a right audio signal received from the second audio input. To clock the composite audio signal to the selected frequency, the audio signal processor uses either a first signal embedded within a primary video signal received from the video/audio signal generated at the video input or a second signal embedded within a secondary video signal generated by an internal video source. A switch selectively couples either the video input or an output of the internal video signal source to the audio signal processor. The audio signal processor clocks the composite audio signal to the frequency of the first signal when the video input is coupled to the audio signal processor and clocks the composite audio signal to the frequency of the second signal when the internal video source is coupled to the audio signal processor.
In one aspect of the invention, the frequency of the second signal is 17.734 KHz, the frequency of the horizontal synchronization signal for video signals to which copy protection has not been applied. If the primary video signal is not copy protected, the audio signal processor will clock the composite audio signal to the frequency of the horizontal synchronization signal embedded therein. If, however, the primary video signal has been copy protected, the frequency of the horizontal synchronization signal embedded therein will have been shifted, for example, to 17.890 KHz. In such circumstances, the switch is used to couple the internal video signal generator to the audio signal processor, thereby ensuring that the composite audio signal will always be clocked to a selected frequency, for example, 17.734 KHz.